This invention relates to hydraulic circuits including a hydraulic motor and provided with an overrunning load control.
Many directional control circuits for high pressure hydraulic systems utilize check valves of the poppet type for controlling the flow of fluid to and from a hydraulic motor. Poppet type check valves provide positive blockage of fluid flow when closed to a greater degree than, for example, conventional spool valves.
In general, the speed of the hydraulic motor is controlled by modulating the fluid flow from a pump to the motor. However, a difficulty is frequently encountered in terms of providing fine control of a so-called "overrunning load" condition as, for example a heavy load supported by hydraulic cylinders being lowered. In such a case, the flow path through the poppet valve must be finely controlled in order to ensure positive, fine and reproducible control of the hydralic motor.
While many systems heretofore designed for the purpose of providing fine control of a hydraulic motor when an overrunning load condition exists have performed generally satisfactorily, system instability may occur because of a valve's response to its internal fluid flow forces or as a result of interactions with other system components. Moreover, in some cases, control characteristics may vary, dependent upon the load itself.